Wavelength division multiplexing optical transmission system and method

ABSTRACT

Disclosed is a wavelength division multiplexing (WDM) optical transmission system including: an optical transmitter for transmitting, to an optical fiber transmission path, a WDM signal obtained by multiplexing a plurality of optical signals on the optical fiber transmission path in terms of wavelength, the plurality of optical signals respectively having negative chirps; an optical receiver for receiving the WDM signal from the optical fiber transmission path; and at least one relay node which is provided between the optical transmitter and the optical receiver. Each of the relay node and the optical receiver includes a dispersion compensator for compensating a chromatic dispersion suffered in the optical fiber in the immediately preceding transmission span. Moreover, a dispersion adder for beforehand adding a predetermined positive dispersion amount to the WDM signal before transmission is included in the optical transmitter or at least one relay node.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength division multiplexingoptical transmission system and a wavelength division multiplexingoptical transmission method. More specifically, the present inventionrelates to a wavelength division multiplexing optical transmissionsystem and a wavelength division multiplexing optical transmissionmethod, both of which make it possible to make a residual dispersionvalue in a received optical signal an optimal value.

2. Description of the Related Art

An “optical transmission” method of transmitting, and receiving,information as a light intensity modulation signal has been used for thepurpose of transmitting a large amount of data in a long distance inrecent years. In the case of the optical transmission method, an opticaltransmitter converts an electrical signal to an optical signal (E/Oconversion), and transmits the optical signal to an optical receiver.The optical receiver converts the received optical signal to anelectrical signal (O/E conversion), and thereby obtains the originalinformation. A communications system using an optical fiber as atransmission path has been generally known as a communications systememploying the optical transmission method. In the case of the opticalfiber communications system, a method of transmitting an optical signalwhich is obtained by multiplexing the signals on a single optical fiberhas been used in order to increase the amount of information to betransmitted in each optical fiber.

A method of multiplexing signals while the signals are still in the formof electrical signals and a method of multiplexing signals after thesignals are converted to optical signals are applicable to the opticaltransmission. As the former method, Time Division Multiplexing (TDM) andFrequency Division Multiplexing (FDM) are known. As the latter method,Space Division Multiplexing (SDM) and Wavelength Division Multiplexing(WDM) are known. Out of these methods, WDM is a method of causing aplurality of optical signals each with a different wavelength to betransmitted with a single optical fiber. Since WDM can use the existingoptical fiber networks, WDM is economically advantageous, and is putinto practical use widely throughout the world.

In the case of the WDM optical transmission system, signals aredeteriorated stemming from chromatic dispersion in the transmission path(optical fiber). In other words, since a luminance element for the E/Oconversion in an optical transmitter is a light source-whose spectrumspreads, each of optical signals to be sent out from the opticaltransmitter actually has some degree of a bandwidth. Optical signalswith the respective different wavelengths (i.e., different frequencies)are different from one another in speed at which each of the opticalsignals travels through the optical fiber. For this reason, as thetransmission distance becomes longer, phases are shifted depending onthe frequency components even in a single optical signal. Thisconstitutes a cause of bit errors.

For this reason, in the WDM optical transmission system, compensation isneeded to return, to zero, chromatic dispersion suffered in the opticalfiber transmission path for the purpose of inhibiting the signal frombeing deteriorated. United States Patent Application Publication No. US2003/0095766A1 has disclosed an optical transmission system in which adispersion compensation fiber is connected to a transmission pathoptical fiber in each span, and in which sufficient dispersioncompensation is performed over the entire optical transmission path.

Descriptions will be provided for an example of a dispersion map of aconventional WDM optical transmission system with reference to FIG. 1Aand FIG. 1B. As shown in FIG. 1A, in this WDM optical transmissionsystem, optical signals from an optical transmitter are transmitted toan optical receiver through some relay nodes. The wavelengths aredispersed in the transmission path optical fiber in each of spansrespectively between the optical transmitter and the nearest relay node,between each two neighboring relay nodes, and between the opticalreceiver and the nearest relay node.

For this reason, the dispersion is compensated by 100% in each of thespans by use of a Dispersion Compensation Fiber (DCF). The DCF has adispersion value which is equal, in absolute value, to the dispersion ofthe span of the transmission path optical fiber, and whose sign isopposite to the dispersion of the span of the transmission path opticalfiber. How the chromatic dispersion is compensated by use of the DCFs isshown in the dispersion map of FIG. 1B. The horizontal axis representsthe transmission distance, and corresponds to each of the transmissionspans shown in FIG. 1A. In the case of this compensation method, the WDMsignal needs to be compensated collectively. For this reason, thedispersion slope of the transmission path optical fiber should beconsidered to compensate the chromatic dispersion by 100% in each of thetransmission spans. With regard to transmission path fibers each with ahigher dispersion slope, including some of Dispersion Shifted Fibers(DSFs), it is difficult for the dispersion to be compensated by 100%over the entire wavelengths of the WDM signal by use of the DCFs.

In addition, an LN (LiNbO3) optical modulator has been generally used asan optical modulator in an optical transmitter in the case where thelong-distance transmission is performed. In general, the LN modulator isoperated in a way that each of the optical signals has a negative chirpcoefficient. The reason for this is that the dispersion is intended tobe easily compensated for the purpose of inhibiting change in waveformsafter performing the optical fiber transmission. The change in waveformsstems from combination of the Self Phase Modulation (SPM) and thechromatic dispersion. For this reason, with regard to the signals to bereceived by the optical receiver through the optical fiber, an optimumresidual dispersion value does not take on zero, but a specific value.Incidentally, the optimum residual dispersion value is defined as thedispersion value which makes the bit error rate the smallest when testpatterns (bit patterns) are sent out from the optical transmitter.

This point will be described in detail. Theoretically, when the residualdispersion value is returned to zero by means of completely compensatingthe dispersion, the signal takes the same waveform as the signal tookwhen the signal was transmitted. However, when the dispersion isprovided thereto, a phenomenon additionally occurs in which the biterror rate is further improved. This phenomenon occurs because of phasemodulation accompanied by intensity modulation called “chirp”. In a casewhere the chirp is present, the optical pulse is compressed due todispersion accumulation. This improves the receiver sensitivity.

The chirp which causes the aforementioned phenomenon is a kind of phasemodulation. The phase modulation includes the chirp which is provideddue to characteristics of the optical transmitter at a time oftransmission. In addition to the chirp, the phase modulation alsoincludes nonlinear phase modulation which is provided due to an opticalnonlinear effect in the transmission path optical fiber. The opticalnonlinear effect is a phenomenon in which the refractive index of anoptical fiber changes depending on the instantaneous optical power. Dueto this phenomenon, the optical signal transmitted through the opticalfiber suffers the phase modulation. The nonlinear phase modulationeffects in the WDM system include Self Phase Modulation (SPM) stemmingfrom its own channel and Cross Phase Modulation (XPM) stemming fromother propagating channels in parallel. These nonlinear phase modulationeffects occur at a moment when an optical signal is made incident ontothe optical fiber transmission path from the optical amplifier. For thisreason, the cumulative amount of the nonlinear phase modulation effectsvaries depending on the launched optical power into the optical fiberand the number of relays (i.e., repeat) of the optical amplifier, etc.

Even if the dispersion compensation fiber is designed to return theresidual dispersion value to zero (achieve the 100% compensation), theamount of the chromatic dispersion varies from one wavelength toanother, as shown in FIG. 2. In other words, the transmission path has adispersion slope. For this reason, in a case where a dispersioncompensation fiber which does not match the compensation dispersionvalue and the transmission path dispersion value with each other isused, it is difficult to cause all of the residual dispersion values ofall the wavelength components to be equal to intended values.

By use of FIG. 3A and FIG. 3B, descriptions will be provided for causesof a compensation error which occurs in a case where a residualdispersion value in the optical receiver is not caused to return to zeroin the conventional WDM optical transmission system. In the transmissionsystem shown in FIG. 3A, it is supposed that the residual dispersionvalue at the optical receiver is the intended particular value otherthan zero as shown in a dispersion map in FIG. 3B. However, it isdifficult to cause all of the residual dispersion values of all thewavelength components to be equal to the intended value, even if theoptical receiver is designed not to compensate the chromatic dispersionin the last span by 100%. In other words, in a case where a dispersioncompensation fiber which does not match the compensation dispersionvalue and the transmission path dispersion value with each other isused, the residual dispersion value of a particular wavelength componentmay be equal to the intended value, but the residual dispersion valuesof the other wavelength components are not equal to the intended values.As a consequence, the residual dispersion in the WDM bandwidth causes acompensation error in response to the dispersion slope in thetransmission path fiber.

Particularly, in a case where the optical fiber as the transmission pathis a DSF, this decreases the rate of compensation of the dispersionslope in the DCF. Accordingly, this brings about a problem of a largecompensation error occurring in the residual dispersion. As describedabove, in the conventional WDM optical transmission system, it has beendifficult to simultaneously achieve the causing of the targeted residualdispersion value (not “zero”) of the received optical signal and thedecreasing of the dispersion compensation error.

Incidentally, another related art have been disclosed in U.S. Pat. No.6,324,317B1 or US Patent Application Publication No. US2002/0101633A1.In the case of these techniques, however, a negative preset dispersionis provided, and a final amount of the accumulated chromatic dispersionis in the negative region. The precondition for these techniques isdifferent from the precondition for the present invention. Thus, nomethod of solving the aforementioned problems has been disclosed inthese related arts.

SUMMARY OF THE INVENTION

The present invention was made in view of the foregoing and otherproblems, drawbacks and disadvantages of the conventional system andmethod.

An exemplary feature of the present invention is to provide a wavelengthdivision multiplexing optical transmission system and a wavelengthdivision multiplexing optical transmission method, both of whichsimultaneously achieve the causing of a targeted residual dispersionvalue of a received optical signal and the decreasing of the dispersioncompensation error.

The present invention provides a wavelength division multiplexingoptical transmission system including: an optical transmitter fortransmitting, to an optical fiber transmission path, a WDM signal whichis obtained by multiplexing a plurality of optical signals on theoptical fiber transmission path in terms of wavelength, the plurality ofoptical signals respectively having negative chirps, and the pluralityof optical signals differing from one another in wavelength; an opticalreceiver for receiving the WDM signal from the optical fibertransmission path; and at least one relay node provided between theoptical transmitter and the optical receiver; wherein the opticaltransmitter includes a dispersion adder for beforehand adding apredetermined positive dispersion amount to the WDM signal before theWDM signal is transmitted; and wherein each of the relay nodes and theoptical receiver includes a dispersion compensator for compensating achromatic dispersion suffered in the optical fiber transmission path inthe immediately preceding transmission span.

Further, the present invention provides a wavelength divisionmultiplexing optical transmission system including: an opticaltransmitter for transmitting, to an optical fiber transmission path, aWDM signal which is obtained by multiplexing a plurality of opticalsignals on the optical fiber transmission path in terms of wavelength,the plurality of optical signals respectively having negative chirps,and the plurality of optical signals differing from one another inwavelength; an optical receiver for receiving the WDM signal from theoptical fiber transmission path; and at least one relay node which isprovided between the optical transmitter and the optical receiver;wherein each of the relay node and the optical receiver includes adispersion compensator for compensating a chromatic dispersion sufferedin the optical fiber in the immediately preceding transmission span; andwherein at least one of the relay nodes further includes a dispersionadder for adding a predetermined positive dispersion amount to the WDMsignal before the WDM signal is transmitted to the optical fiber in thefollowing transmission span.

Further, the present invention provides a wavelength divisionmultiplexing optical transmission method in a wavelength divisionmultiplexing optical transmission system including an opticaltransmitter, an optical receiver and at least one relay node which isprovided between the optical transmitter and the optical receiver, thetransmission method comprising the steps of: causing the opticaltransmitter to add a predetermined positive dispersion amount to a WDMsignal which is obtained by multiplexing a plurality of optical signalson the optical fiber transmission path in terms of wavelength, and tothus transmit the WDM signal to an optical fiber transmission path, theplurality of optical signals modulated by an optical modulator so as torespectively have negative chirps, and the plurality of optical signalsdiffering from one another in wavelength; causing each of the relaynodes to compensate a chromatic dispersion, which the received WDMsignal has suffered in the optical fiber in the immediately precedingtransmission span, and to thus transmit the WDM signal to thetransmission path in the following transmission span; and causing theoptical receiver to compensate a chromatic dispersion, which thereceived WDM signal has suffered in the optical fiber in the immediatelypreceding transmission span, and to thereby obtain the received WDMsignal, which includes the predetermined positive dispersion amount.

Further, the present invention provides a wavelength divisionmultiplexing optical transmission method in a wavelength divisionmultiplexing optical transmission system including an opticaltransmitter, an optical receiver and at least one relay node which isprovided between the optical transmitter and the optical receiver, thetransmission method comprising the steps of: causing the opticaltransmitter to transmit, to an optical fiber transmission path, a WDMsignal which is obtained by multiplexing a plurality of optical signalson the optical fiber transmission path in terms of wavelength, theplurality of optical signals respectively having negative chirps, andthe plurality of optical signals differing from one another inwavelength; causing each of the relay nodes to compensate a chromaticdispersion, which the received WDM signal has suffered in the opticalfiber in the immediately preceding transmission span, and to thustransmit the WDM signal to the transmission path in the followingtransmission span; causing at least one of the relay nodes to furtheradd a predetermined positive dispersion amount to the WDM signal beforethe WDM signal is transmitted to the optical fiber in the followingtransmission span; and causing the optical receiver to compensate achromatic dispersion, which the received WDM signal has suffered in theoptical fiber in the immediately preceding transmission span, and tothereby obtain the received WDM signal, which includes the predeterminedpositive dispersion amount.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other exemplary aspects, features and advantages of thepresent invention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawingswherein:

FIG. 1A and FIG. 1B are diagrams showing a dispersion map of chromaticdispersion in a conventional WDM optical transmission system,

FIG. 2 is a graph showing a dispersion slope of an optical fiber,

FIG. 3A and FIG. 3B are diagrams showing a dispersion map in a casewhere a residual dispersion value in an optical receiver is not returnedto zero in the conventional WDM optical transmission system,

FIG. 4A and FIG. 4B are diagrams showing a configuration of a WDMoptical transmission system according to a first exemplary embodiment ofthe present invention,

FIG. 5A and FIG. 5B are diagrams showing a dispersion map of a residualdispersion value in the WDM optical transmission system according to thefirst exemplary embodiment of the present invention,

FIG. 6 is a graph showing a residual dispersion value in each node in acase where an LN modulator is designed to take on a positive chirp,

FIG. 7 is a graph showing a residual dispersion value in each node in acase where an LN modulator is designed to take on a negative value,

FIG. 8A and FIG. 8B are diagrams showing how a WDM signal variesdepending on whether the chirp used in LN modulation takes on a positivevalue or on a negative value,

FIG. 9A and FIG. 9B are diagrams showing how influence of nonlineareffects varies depending on whether the chirp used in LN modulationtakes on a positive value or on a negative value,

FIG. 10 is a diagram showing a configuration of a WDM opticaltransmission system according to a second exemplary embodiment of thepresent invention,

FIG. 11 is a diagram showing a configuration of an OADM according to thesecond exemplary embodiment of the present invention,

FIG. 12 is a diagram showing a configuration of a λ module shown in FIG.11,

FIG. 13A and FIG. 13B are diagrams showing the reason why a differencein dispersion value occurs between the through signal and an ADD signalin the OADM in a case where the residual dispersion value at the opticalreceiver is not zero in the WDM optical transmission system,

FIG. 14A and FIG. 14B are diagrams showing a dispersion map of aresidual dispersion value in the second exemplary embodiment of thepresent invention,

FIG. 15 is a diagram showing a configuration of a WDM opticaltransmission system according to a third exemplary embodiment of thepresent invention,

FIG. 16 is a diagram showing a configuration of a WDM opticaltransmission system according to a fourth exemplary embodiment of thepresent invention, and

FIG. 17A and FIG. 17B are diagrams showing a dispersion map of aresidual dispersion value in the fourth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First of all, descriptions will be provided for a theory for the presentinvention. In the present invention, a positive preset dispersion valueis beforehand given to the WDM signal in any one of the opticaltransmitter and the relay nodes, and thereafter the WDM signal thusprovided with the positive dispersion value is transmitted to theoptical receiver through the optical fiber transmission path. Thereby,only if the chromatic dispersion is compensated by 100% in each span inthe transmission path, the signal received by the optical receiver canbe the signal in which an optimum dispersion value (approximately tensps/nm to approximately thousands ps/nm; and most advantageously,approximately not smaller than +100 ps/nm but smaller than +500 ps/nm inparticular) remains. Hereinafter, descriptions will be provided for theexemplary embodiments of the present invention based on theaforementioned theory.

First Exemplary Embodiment

By use of FIG. 4A, descriptions will be provided for a configuration ofa WDM optical transmission system according to the first exemplaryembodiment of the present invention. This WDM optical transmissionsystem has a configuration in which an optical transmitter 1 and anoptical receiver 2 are connected with each other through a transmissionpath optical fiber 3 (spans 3 a, 3 b, 3 c, . . . ) and relay nodes 4 (4a, 4 b, . . . ).

The optical transmitter 1 includes a plurality of transponders 11, anOptical Multiplexer (OMUX) 12, a positive-dispersion adder 13 and anoptical amplifier 14. The plurality of transponders (TPND) 11 correspondrespectively to wavelengths to be multiplexed, as a WDM signal (forexample, in the wavelength range of 1.5 μm), on the transmission pathoptical fiber.

The transponders 11 receive a signal (data) to be transmitted from aclient terminal (not illustrated), and modulates the light intensity byuse of an LN (LiNbO3) modulator on the basis of the signal thusreceived. Then, the transponders 11 output to the OMUX 12 the signal,light intensity of which is thus modulated. In a case where the datafrom the client is in the form of an optical signal, the optical signalsare once converted to an electric signal through O/E conversion. Asshown in FIG. 4B, each of the transponders 11 includes inside at least apublicly-known LN modulator 110 and a light source 111 of wavelengths tobe transmitted. Data signals to be transmitted are inputted into the LNmodulator 110. Each of the transponders 110 further includes aconfiguration needed to control operations of its own, although theconfiguration is not illustrated in detail.

The OMUX 12 multiplexes the optical signals inputted respectively fromthe transponders 11. The OMUX 12 can be realized, for example, by use ofArrayed Waveguide Grating (AWG). The positive-dispersion adder 13 adds apositive dispersion to the optical signals. In the present invention,the LN modulator is controlled by use of a negative chirp, as describedbelow. It was found that the aforementioned optimum residual dispersionvalue “X” for the present invention was +100 ps/nm to +500 ps/nm. Inthis embodiment, therefore, the positive-dispersion adder 13 adds adispersion value X of +100 ps/nm to +500 ps/nm, as the target optimumresidual dispersion value, to the optical signals. A photonic crystalfiber, a single mode fiber and the like can be applied for thepositive-dispersion adder 13. With regard to the single mode fiber, morespecifically, 1.3 μm zero-dispersion single mode fiber can be appliedfor the positive-dispersion adder 13. The optical amplifier 14 amplifiesthe WDM optical signal.

The optical receiver 2, includes an optical amplifier 21, a dispersioncompensator 22, an Optical De-Multiplexer (ODMUX) 23 and a plurality oftransponders (TPND) 24. The plurality of transponders 24 correspondrespectively to wavelengths of the WDM signal. The optical amplifier 21amplifies the optical signals to be transmitted thereinto through theimmediately preceding span 3 c of the optical fiber. The dispersioncompensator 22 can be realized, for example, by use of a DispersionCompensation Fiber (DCF). The DCF 22 compensates a chromatic dispersionsuffered in the optical fiber 3 c between the optical receiver 2 and itsimmediately preceding relay node 4 b. The ODMUX 23 separates thereceived WDM optical signals to the respective channels (wavelengths),and outputs the WDM optical signals thus separated respectively to thetransponders 24 corresponding to the wavelengths. The ODMUX 23 can berealized by use of Arrayed Waveguide Grating (AWG), for example. Thetransponders 24 detect the optical signals received from the ODMUX 23.For example, in the transponder 24, an optical receiving device detectswhether or not an optical signal with the specific wavelength has beenreceived through a bandpass filter corresponding the wavelength. Thetransponder 24 outputs the received signals to the client terminal (notillustrated). In the case where the signals are intended to beoutputted, in the form of optical signals, to the client terminal, thesignals are once converted to the electrical signals, and thereafter areonce again converted to the optical signals each with a desiredwavelength.

A dispersion shifted fiber, a 1.3 μm zero-dispersion single mode fiber,a non-zero dispersion shifted fiber and the like can be applied for theoptical fibers 3 a, 3 b and 3 c respectively in the spans constitutingthe transmission path optical fiber 3.

The relay node 4 a includes an optical amplifier 41 a and a dispersioncompensator to be realized by use of a Dispersion Compensation Fiber(DCF) 42 a. The optical amplifier 41 a amplifies the optical signals tobe transmitted thereinto through the optical fiber 3 a. The DCF 42 acompensates the chromatic dispersion suffered in the optical fiber 3 abetween the relay node 4 a and the optical transmitter 1. Another relaynode 4 b includes an optical amplifier 41 b and a DCF 42 b, in commonwith the relay node 4 a. Only the two relay nodes have been illustratedin FIG. 4A. However, the number of transmission spans and relay nodes tobe used in a real situation is determined depending on the totaltransmission distance.

In this embodiment, the relay nodes 4 a and 4 b in the transmission pathrespectively compensate, by 100%, the chromatic dispersions in the spans(the optical fibers 3 a and 3 b). A dispersion map of the residualdispersion value is shown in FIG. 5A and FIG. 5B. By use of FIG. 4A,FIG. 5A and FIG. 5B, descriptions will be provided for operations of theWDM optical transmission system according to this embodiment. The WDMsignal added the optimum dispersion value (preset dispersion value X) bythe positive-dispersion adder 13 is outputted from the opticaltransmitter 1 to the transmission path directed towards the opticalreceiver 2. The chromatic dispersion (dispersion suffered by thetransmission path) begins to occur in the transmission path. Then, thedispersion value of the WDM signal increases gradually. When the WDMsignal reaches each of the relay nodes 41 a and 41 b, the chromaticdispersion introduced by the transmission path is compensated by 100%respectively by the DCFs 42 a and 42 b (i.e., the dispersion value inthe WDM signal returns to the preset dispersion value X). In otherwords, the chromatic dispersion of the WDM signal is caused to decreaseby the chromatic dispersion suffered by the transmission path which hasoccurred in the immediately preceding optical fiber 3 a and 3 b.Thereby, the WDM signal outputted from one relay node to the next relaynode includes the dispersion equal to the preset dispersion X valueadded in the optical transmitter 1. Similarly, the WDM signal outputtedfrom a relay node immediately preceding the optical receiver 2 includesthe dispersion equal to the preset dispersion value. The dispersionoccurred in the transmission path fiber between the optical receiver 2and the immediately preceding relay node is also compensated by 100% byuse of the DCF 22 in the optical receiver 2. Thereby, finally, only thepreset dispersion value added in the positive-dispersion adder 13 in theoptical transmitter 1 remains in the received WDM-signal.

The optical signals transmitted from the transponders 11 in the opticaltransmitter 1 are defined as negative chirps which makes it easy toperform dispersion management on waveform deterioration stemming fromchromatic dispersion and nonlinear effects during multi-spantransmission. Additional descriptions will be provided for the chirp inthe LN modulator in the optical transmitter.

FIG. 6 shows a relationship between the input optical power into theoptical fiber and an optimum residual dispersion value to be received,which relationship is observed in a case where transmission is performedat a bit rate of 10 Gbit/s over a transmission distance of DSF 80 km×6spans while the LN modulator is caused to operate with positive chirp.As shown in FIG. 6, in the case where the LN modulator is caused tooperate with positive chirp, the dispersion compensation in each of therelay nodes can not be achieved accurately due to a combination of selfphase modulation and chromatic dispersion suffered in the transmissionpath. Furthermore, a fluctuation of the inputted power into the opticalfiber transmission path is also caused by the combination. If theinputted power into the optical fiber transmission path is fluctuated,this leads to fluctuation of the optimum residual dispersion value atthe optical receiver 2.

FIG. 7 shows a relationship between the input optical power into theoptical fiber and an optimum residual dispersion value to be received,which relationship is observed in a case where transmission is performedat a bit rate of 10 Gbit/s over a transmission distance of DSF 80 km×6spans while the LN modulator is caused to operate with negative chirp.As shown in FIG. 7, in the case where the chirps of the optical signalsare caused to be “negative”, the optimum residual dispersion value iskept virtually constant regardless of the dispersion map or the incidentpower onto the optical fiber.

By use of FIG. 8A and FIG. 8B, more detailed descriptions will beprovided for influence of the difference in chirp between “positive” and“negative”. In the case of the positive chirp, the shorter wavelengthtravels slower, and the longer wavelength travels faster. For thisreason, the WDM signal is dispersed widely as shown in FIG. 8A. On thecontrary, in the case of the negative chirp, the shorter wavelengthtravels faster, and the longer wavelength travels slower. For thisreason, the WDM signal is compressed around the center wavelengthcomponent as shown in FIG. 8B. As a result, the signal-to-noise ratio(SNR) is high in the case of the negative chirp.

FIG. 9A and FIG. 9B show influence of the nonlinear effects which isobserved in conjunction with change in the input power into the opticalfiber. As the input power into the optical fiber increases, thenonlinear effects have a stronger influence. The nonlinear effects causethe shorter wavelengths to travel faster, and cause the longerwavelengths to travel shorter, like as in the case of the negativechirps. For this reason, in the case of the positive chirp, increase ofthe input power into the optical fiber makes it likely that the chirp isactually inverted between positive and negative (as shown in FIG. 9A).

This causes the signal waveform to largely change in conjunction withthe change in the input power into the optical fiber in the case of thepositive chirp. Accordingly, the optimum residual dispersion valuechanges from negative to positive to a large extent, as shown in FIG. 6.On the contrary, in the case of the negative chirp, no substantial chirpinversion is observed even if the input power into the optical fiberchanges. For this reason, change in the signal waveform is small (asshown in FIG. 9B). As a consequence, the optimum residual dispersionvalue is a substantially constant value, as shown in FIG. 7. From this,it is found that, advantageously, the optical signals are transmitted bymeans of exercising control so as to cause the optical signals to havethe respective negative chirps. In the case of each of the exemplaryembodiments of the present invention, the optical signals outputted fromthe LN modulator which is the optical modulator have the respectivenegative chirps.

Thereby, the residual dispersion value of the signals received in theoptical receiver can be an optimum value, only if the chromaticdispersion generated by the transmission path is compensated by 100% ineach span.

In this embodiment, the residual dispersion value of the signal receivedin the optical receiver can be an optimum value in this manner, even ifthe dispersion map designed to compensate the chromatic dispersion by100% in each span is used.

Particularly in a case where a DSF is used as the transmission pathoptical fiber, the dispersion slope compensation ratio of the DSF islow. This has heretofore brought about a problem of causing-a largecompensation error in the residual dispersion. In the present invention,however, even if the DSF is used for the transmission path, the causingof the residual dispersion value of the received optical signal to bethe optimum value other than zero and the decreasing of the dispersioncompensation error can be achieved simultaneously.

Second Exemplary Embodiment

Next, by use of FIG. 10, descriptions will be provided for aconfiguration of a WDM optical transmission system according to a secondexemplary embodiment of the present invention. This WDM opticaltransmission system has a configuration in which an optical transmitter1 and an optical receiver 2 are connected with each other through anoptical fiber 3 (spans 3 a, 3 b, 3 c, . . . ), a relay node 4 and anOptical Add Drop Multiplexer (OADM) node 5. The optical transmitter 1,the optical receiver 2, the optical fiber 3 and the relay node 4respectively have the same configurations as those according to thefirst embodiment have.

The OADM node 5 includes an optical amplifier 51, a dispersioncompensator using a Dispersion Compensation Fiber (DCF) 52, an OADM 53,a transponder 54, a positive-dispersion adder 55 and an opticalamplifier 56.

The optical amplifier 51 amplifies an optical signal from the relay node4 through the optical fiber 3 b. The DCF 52 compensates the chromaticdispersion suffered in the optical fiber 3 b. The OADM 53 has a DROPfunction and an ADD function. The DROP function is to drop only anarbitrary wavelength channel from the received WDM optical signal, andto output the dropped wavelength channel to the transponder 54. The ADDfunction is to add an optical signal from the transponder 54 to the WDMoptical signal.

The ADD function carried out by the transponder 54 is to receive signals(data) from a client terminal (not illustrated), and modulates the lightintensity by use of an LN modulator on the basis of the receivedsignals. Then, the transponder 54 outputs the modulated optical signalsto the positive-dispersion adder 55. In a case where the data from theclient is in the form of an optical signal, the optical signals are onceconverted to electric signals through O/E conversion. The LN modulatorperforms the same operations as the LN modulator described in FIG. 4B.Each of the modulated optical signals has a negative chirp. The DROPfunction performed by the transponder 54 is to receive, and to detect,the optical signal which the OADM 53 has dropped from the WDM signal.For example, an optical receiving device detects whether or not anoptical signal with the arbitrary wavelength has been received through abandpass filter corresponding to the wavelength. The transponder 54outputs the received signals to the client terminal (not illustrated).In the case where the signals are intended to be outputted, in the formof optical signals, to the client terminal, the signals are onceconverted to the electrical signals, and thereafter are once againconverted to the optical signals each with a desired wavelength.

The positive-dispersion adder 55 adds a positive preset dispersion tothe optical signal which has been received from the transponder 54, andoutputs to the OADM 53 the optical signal with the positive dispersionadded thereto. In this embodiment, the dispersion value is in range of+100 ps/nm to +500 ps/nm, as described above. The positive-dispersionadder 55 is designed to add a dispersion value equal to the addeddispersion value X by the positive-dispersion adder 13 in the opticaltransmitter 1. The optical amplifier 51 amplifies the WDM optical signaloutputted by the OADM 53.

FIG. 11 shows an example of a configuration of the OADM 53. At least oneλ module 531, which is detachable, is attached to the OADM 53. The λmodule 531 is designed to correspond to any one wavelength of thewavelength components in the WDM signal. The λ module 531 drops, fromthe WDM signal, optical signal with a wavelength to which the λ module531 corresponds, and superimposes on the WDM signal an ADD signal towhich the λ module 531 corresponds. For example, in a case where the WDMsignal is constituted of wavelength components λ1 to λ20, the λ module531 corresponding to the wavelength λ1 drops the wavelength component λ1from the WDM signal, and lets the wavelength components λ2 to λ20 passthrough it. In addition, the λ module 531 corresponding to thewavelength λ1 superimposes an optical signal (ADD signal) constituted ofthe wavelength λ1 onto the WDM signal (through signal) constituted ofthe wavelength components (λ2 to λ20) which the λ module 531 let passthrough it. Incidentally, the configuration in which the OADM 53includes the λ modules 531 has been given as the example of theconfiguration of the OADM 53 here. However, no matter what configurationmay be used for the OAMD 53, if the configuration enables a desiredwavelength component to be added to, and dropped from, the WDM signal.The OADM 53 may take a publicly-known configuration in which an opticalswitch or a wavelength blocker is used, for example.

FIG. 12 shows an example of a configuration of the λ module. The λmodule 531 includes a circulator 5311, a fiber grating 5312 and anoptical coupler 5313. The circulator 5311 guides to the fiber grating5312 the WDM signal inputted from the outside, and drops an opticalsignal reflected by the fiber grating 5312.

The fiber grating 5312 reflects only a wavelength component, which the λmodule 531 corresponds to, out of the wavelength components of the WMDsignal from the circulator 5311, and sends back to the circulator 5311the reflected wavelength component. The fiber grating 5312 lets theother wavelength components pass through it, and sends out the passingwavelength components to the optical coupler 5313. The optical coupler5313 superimposes the wavelength components of the WDM signal, which thefiber grating 5312 let pass through it, and the ADD signal on oneanother.

For example, it is supposed that the λ module 531 of FIG. 12 correspondsto a wavelength component λ1 and the WDM signal inputted from theoutside is constituted of wavelength components λ1 to λ4. The inputtedWDM signal is guided to the fiber grating 5312 by the circulator 5311.The fiber grating 5312 reflects only an optical signal with thewavelength λ1, and sends the reflected optical signal back to thecirculator 5311. The fiber grating 5312 lets optical signals with therespective wavelengths λ2 to λ4 pass through it. On the other hand, inthe optical coupler 5313, the ADD signal with the wavelength λ1 issuperimposed on the wavelength components λ2 to λ4 passed through thefiber grating 5312. Thereby, the WDM signal constituted of thewavelength components λ1 to λ4 is outputted from the λ module 531.

A WDM optical transmission system including an OADM node brings about aproblem in addition to the above-described problems with the relatedart, in a case where the residual dispersion value of the optical signaldropped in the OADM node is caused to be the optimum value other thanzero. By use of FIG. 13A and FIG. 13B, descriptions will be provided forthe additional problem giving an example of no preset dispersion beingadded. Specifically, as shown in FIG. 13B, there is difference inresidual dispersion amount between the components to pass through theOADM node in the WDM signal and the optical signal to be added in theOADM node.

In this embodiment, the LN modulator in each of the transponders 11 inthe optical transmitter 1 operates with negative chirp which bringsabout relatively small nonlinear effects during multi-span transmission.Furthermore, a specific preset positive dispersion value is given to theWDM optical signal outputted from the optical transmitter 1 and the ADDsignal in the OADM node 5.

Thereby, if chromatic dispersion generated by the transmission path iscompensated by 100% in each span, this makes the residual dispersionvalue of the signal to be received by the optical receiver 2 an optimumvalue, as shown in FIG. 14A and FIG. 14B. Moreover, this makes thedispersion value in the wavelength components to pass through the OADMnode 5 in the WDM optical signal agree with the dispersion value of theADD signal in the OADM node 5.

According to this embodiment, even if the dispersion map designed tocompensate the chromatic dispersion by 100% in each span is used, theresidual dispersion value of the received signal at the optical receiver2 can be made an optimum value in the aforementioned manner. As well,even if the OADM node is inserted into the transmission path, theoptical receiver 2 can receive the WDM signal which includes the optimumresidual dispersion value.

Third Exemplary Embodiment

By use of FIG. 15, descriptions will be provided for a WDM opticaltransmission system according to a third exemplary embodiment of thepresent invention. This WDM optical transmission system has aconfiguration in which an optical transmitter 10 and the opticalreceiver 2 are connected with each other through the optical fiber 3(spans 3 a, 3 b, 3 c, . . . ) and the relay nodes 4 (4 a, 4 b, . . . ),in common with the WDM optical transmission system according to thefirst embodiment. The optical receiver 2, the optical fiber 3 and therelay nodes 4 are identical to those included in the first embodiment.However, the optical transmitter 10 is different in configuration fromthe optical transmitter included in the first embodiment. In thisembodiment, the optical transmitter 10 includes no positive-dispersionadder 13, but instead a plurality of positive-dispersion adders 15. Theother parts of the configuration, which are denoted by the samereference numerals as those in the first embodiment, are the same asthose in FIG. 4A.

The transponders 11 in this embodiment may be the same as those whichhave been described with regard to the first embodiment. However,transponders 11 in this embodiment output an optical signal to therespective positive-dispersion adders 15. Each of thepositive-dispersion adders 15 gives a positive/preset dispersion value Xto the optical signal, and outputs the optical signal to the OMUX 12. Adispersion value added by each of the positive-dispersion adders 15 isequal to the optimum residual dispersion value at the optical receiver2. In this embodiment, the added dispersion value is in a range of +100ps/nm to +500 ps/nm. The OMUX12 multiplexes the optical signals inputtedfrom each of the positive-dispersion adders 15, as described above.

In this embodiment, a preset dispersion value can be individually addedto each wavelength component of the WDM signal. Thereby, the positivedispersion value can be added to each wavelength component more evenly.Specifically, if the residual dispersion value suitable for eachwavelength component of the WDM signal is given to the wavelengthcomponent, this makes it possible to obtain a higher-qualitytransmission characteristic (lower bit error rate). In this embodiment,too, if the WDM signal sent out from the optical transmitter 1 istransmitted to the optical receiver 2 while the WDM signal is beingcompensated by 100% from one relay node 4 to another, this makes itpossible to make the residual dispersion value at the optical receiver 2an optimum value.

Fourth Exemplary Embodiment

By use of FIG. 16, descriptions will be provided for a WDM opticaltransmission system according to a fourth exemplary embodiment. This WDMoptical transmission system has a configuration in which an opticaltransmitter 1 a and the optical receiver 2 are connected with each otherthrough the optical fiber 3 (spans 3 a, 3 b and 3 c) and the relay nodes40 and 4 b. The optical receiver 2 and the optical fiber 3 are identicalto those included in the first embodiment. However, the opticaltransmitter 1 a and the relay node 40 are different in configurationfrom those included in the first embodiment.

The transmitter 1 a in this embodiment includes no positive-dispersionadder 13. Accordingly, the WDM signal outputted from the OMUX 12 isinputted directly into the optical amplifier 14. In addition, the relaynode 40 includes a positive-dispersion adder 43 a in a stage followingthe optical amplifier 41 a. The positive-dispersion adder 43 a adds tothe WMD signal a preset dispersion value X equal to the optimum residualdispersion value for the optical receiver 2. In this embodiment, theadded positive dispersion value is in a range of +100 ps/nm to +500ps/nm.

FIG. 17A and FIG. 17B show a dispersion map of the WDM opticaltransmission system according to this embodiment. The WDM signal with“0” dispersion is sent out from the optical transmitter 1 a to the relaynode 40 through the optical fiber 3 a. With regard to the WDM signalinputted into the relay node 40, the DCF 42 a compensates by 100% thechromatic dispersion which the WDM signal has suffered in the opticalfiber 3 a. The compensated WDM signal is amplified, by the opticalamplifier 41 a, to a signal strength large enough for the WDM signal tobe transmitted to the following relay node 4 b. The positive-dispersionadder 43 a adds a positive dispersion value to the amplified WDM signal.Thereafter, the WDM signal is sent out to the relay node 4 b through theoptical fiber 3 b. In the relay node 4 b, the DCF 42 b compensates by100% the chromatic dispersion suffered in the optical fiber 3 b.Subsequently, the compensated WDM signal is amplified by the opticalamplifier 41 b, and is sent out to the optical receiver 2.

The optimum dispersion at the optical receiver 2 remains in the WDMsignal reached the optical receiver 2 in this manner. For this reason,in the case of this embodiment, too, if the WDM signal sent out from theoptical transmitter 1 is transmitted to the optical receiver 2 while theWDM signal is being compensated by 100% from one relay node 4 toanother, this makes the residual dispersion value at the opticalreceiver 2 an optimum value.

The WDM optical transmission system according to this embodiment hasbeen described giving the case where the optimum dispersion value forthe optical receiver 2 is added to the WDM signal in the relay node 40.It should be noted, however, that this dispersion value may be added tothe WDM signal in the relay node 4 b instead. In other words, thedispersion value may be added to the WDM signal in an arbitrary relaynode provided to the transmission path (optical fiber 3) between theoptical transmitter 1 a and the optical receiver 2.

Furthermore, the number of relay nodes to add preset dispersion value isnot necessarily limited to one. For example, in a case where the optimumresidual dispersion value for the optical receiver 2 is +300 ps/nm,dispersion values of +200 ps/nm and +100 ps/nm may be added to the WDMsignal respectively in the relay node 40 and the following relay node 4b.

In the case of the WDM optical transmission system according to thisembodiment, the optimum dispersion value is added to the WDM signal inthe relay node(s) in the transmission path. For this reason, even if theoptical transmitter has the same configuration as the conventionaloptical transmitter has, the optical receiver receives the WDM signal inwhich the optimum dispersion value remains. As a result, only if therelay node in the conventional WDM optical transmission system isreplaced with the relay node for the WDM optical transmission systemaccording to this embodiment, the effects of the present invention canbe obtained even from the conventional WDM optical transmission system.Hence, even in a case where the conventional optical transmitter can notbe replaced with the optical transmitter system according to the presentinvention, the WDM signal in which the optimum dispersion value iscaused to remain can be received by the receiving terminal.

In addition, the WDM optical transmission system according to thepresent invention may be a system which is obtained by combining thefirst and the fourth embodiments. In other words, the preset dispersionvalue may be added to the WDM signal in the optical transmitter, thedispersion value may be additionally added to the WDM signal in anarbitrary relay node, and thereby the optimum residual dispersion valuemay be designed to be obtained finally at the optical receiver.

It should be noted, moreover, that the relay nodes are not always neededif the transmission distance is short, although the first, the secondand the third embodiments have been described giving examples ofconfigurations having the relay nodes being provided to the transmissionpath.

As well, the configuration including the OADM node which has beendescribed with regard to the second embodiment can be combined with eachof the WDM optical transmission systems according to the third and thefourth embodiments. In the case, however, where the configurationincluding the OADM is combined with the WDM optical transmission systemaccording to the fourth embodiment, the OADM node adds to the ADD signalthe dispersion which is equal to that added to the though signal.

While this invention has been described in connection with certainpreferred embodiments, it is to be understood that the subject matterencompassed by way of this invention is not to be limited to thosespecific embodiments. On the contrary, it is intended for the subjectmatter of the invention to include all alternative, modification andequivalents as can be included within the spirit and scope of thefollowing claims.

Further, it is the inventor's intention to retain all equivalents of theclaimed invention even if the claims are amended during prosecution.

1. A wavelength division multiplexing optical transmission systemcomprising: an optical transmitter for transmitting, to an optical fibertransmission path, a WDM signal which is obtained by multiplexing aplurality of optical signals on the optical fiber transmission path interms of wavelength, the plurality of optical signals respectivelyhaving negative chirps, and the plurality of optical signals differingfrom one another in wavelength; an optical receiver for receiving theWDM signal from the optical fiber transmission path; and at least onerelay node provided between the optical transmitter and the opticalreceiver, wherein the optical transmitter includes a dispersion adderfor beforehand adding a predetermined positive dispersion amount to theWDM signal before the WDM signal is transmitted, and wherein each of therelay nodes and the optical receiver includes a dispersion compensatorfor compensating a chromatic dispersion suffered in the optical fibertransmission path in the immediately preceding transmission span.
 2. Thewavelength division multiplexing optical transmission system accordingto claim 1, wherein the dispersion adder in the optical transmitter addsthe predetermined dispersion amount collectively to each wavelengthcomponent of the WDM signal.
 3. The wavelength division multiplexingoptical transmission system according to claim 2, wherein a photoniccrystal fiber is used for the dispersion adder.
 4. The wavelengthdivision multiplexing optical transmission system according to claim 2,wherein a single-mode fiber is used for the dispersion adder.
 5. Thewavelength division multiplexing optical transmission system accordingto claim 1, wherein the dispersion adder in the optical transmitter addsthe predetermined dispersion amount individually to each wavelengthcomponent in the WDM signal.
 6. The wavelength division multiplexingoptical transmission system according to claim 5, wherein a photoniccrystal fiber is used for the dispersion adder.
 7. The wavelengthdivision multiplexing optical transmission system according to claim 5,wherein a single-mode fiber is used for the dispersion adder.
 8. Thewavelength division multiplexing optical transmission system accordingto claim 1, wherein at least one of the relay nodes further includes: anadditional-signal dispersion adder for adding the predetermined positivedispersion amount to an additional optical signal, which is an opticalsignal with a predetermined wavelength having a negative chirp, andwhich is superimposed on the WDM signal; and an optical-signal adder forsuperimposing the additional optical signal, which the predetermineddispersion amount is added to, on the WDM signal received from theimmediately preceding transmission span.
 9. The wavelength divisionmultiplexing optical transmission system according to claim 8, wherein aphotonic crystal fiber is used for the dispersion adder in the opticaltransmitter and the additional-signal dispersion adder in the relaynode.
 10. The wavelength division multiplexing optical transmissionsystem according to claim 8, wherein a single-mode fiber is used for thedispersion adder in the optical transmitter and the additional-signaldispersion adder in the relay node.
 11. The wavelength divisionmultiplexing optical transmission system according to claim 1, whereineach optical signal in the optical transmitter is modulated by an LNoptical modulator which operates with a negative chirp coefficient. 12.The wavelength division multiplexing optical transmission systemaccording to claim 1, wherein the predetermined dispersion amount is notsmaller than +100 ps/nm, and smaller than +500 ps/nm.
 13. The wavelengthdivision multiplexing optical transmission system according to claim 1,wherein the optical fiber transmission path is a dispersion shiftedfiber.
 14. A wavelength division multiplexing optical transmissionsystem comprising: optical transmission means for transmitting, to anoptical fiber transmission path, a WDM signal which is obtained bymultiplexing a plurality of optical signals on the optical fibertransmission path in terms of wavelength, the plurality of opticalsignals respectively having negative chirps, and the plurality ofoptical signals differing from one another in wavelength; opticalreception means for receiving the WDM signal from the optical fibertransmission path; and at least one relay node means which is providedbetween the optical transmission means and the optical reception means,wherein the optical transmission means includes dispersion adding meansfor beforehand adding a predetermined positive dispersion amount to theWDM signal before the WDM signal is transmitted, and wherein each of therelay node means and the optical reception means includes dispersioncompensating means for compensating a chromatic dispersion suffered inthe optical fiber in the immediately preceding transmission span. 15.The wavelength division multiplexing optical transmission systemaccording to claim 14, wherein at least one of the relay node meansfurther includes: additional-signal dispersion adding means for addingthe predetermined positive dispersion amount to an additional opticalsignal, which is an optical signal with a predetermined wavelengthhaving a negative chirp, and which is superimposed on the WDM signal;and optical-signal adding means for superimposing the additional opticalsignal, which the predetermined dispersion amount is added to, on theWDM signal received from a transmission span immediately preceding theoptical-signal adding means.
 16. A wavelength division multiplexingoptical transmission method in a wavelength division multiplexingoptical transmission system including an optical transmitter, an opticalreceiver and at least one relay node which is provided between theoptical transmitter and the optical receiver, the transmission methodcomprising the steps of: causing the optical transmitter to add apredetermined positive dispersion amount to a WDM signal which isobtained by multiplexing a plurality of optical signals on the opticalfiber transmission path in terms of wavelength, and to thus transmit theVDM signal to an optical fiber transmission path, the plurality ofoptical signals modulated by an optical modulator so as to respectivelyhave negative chirps, and the plurality of optical signals differingfrom one another in wavelength; causing each of the relay nodes tocompensate a chromatic dispersion, which the received WDM signal hassuffered in the optical fiber in the immediately preceding transmissionspan, and to thus transmit the WDM signal to the transmission path inthe following transmission span; and causing the optical receiver tocompensate a chromatic dispersion, which the received WDM signal hassuffered in the optical fiber in the immediately preceding transmissionspan, and to thereby obtain the received WDM signal, which includes thepredetermined positive dispersion amount.
 17. The wavelength divisionmultiplexing optical transmission method according to claim 16, wherein,in the step of causing the optical transmitter to add the predeterminedpositive dispersion amount to the WDM signal, the predetermined positivedispersion amount is added collectively to each wavelength component ofthe WDM signal.
 18. The wavelength division multiplexing opticaltransmission method according to claim 16, wherein, in the step ofcausing the optical transmitter to add the predetermined positivedispersion amount to the WDM signal, the predetermined positivedispersion amount is added individually to each wavelength component ofthe WDM signal.
 19. The wavelength division multiplexing opticaltransmission method according to claim 16, wherein at least one of therelay nodes further includes the steps of: adding the predeterminedpositive dispersion amount to an additional optical signal, which is anoptical signal with a predetermined wavelength having a negative chirp,and which is superimposed on the WDM signal; and superimposing theadditional optical signal, which the predetermined positive dispersionamount is added to, on the WDM signal received from an immediatelypreceding transmission span, in addition to the step of causing therelay nodes to transmit the WDM signal, whose chromatic dispersion iscompensated, to the transmission path.
 20. The wavelength divisionmultiplexing optical transmission method according to claim 16, wherein,in the step of causing the optical transmitter to add the predeterminedpositive dispersion amount to the WDM signal, each optical signal ismodulated by an LN optical modulator which operates with a negativechirp coefficient.
 21. The wavelength division multiplexing opticaltransmission method according to claim 16, wherein, in the step ofcausing the optical transmitter to add the predetermined positivedispersion amount to the WDM signal, each predetermined positivedispersion amount is not smaller than +100 ps/nm, and smaller than +500ps/nm.
 22. The wavelength division multiplexing optical transmissionmethod according to claim 16, wherein, in the step of causing theoptical transmitter to add the predetermined positive dispersion amountto the WDM signal, the predetermined positive dispersion amount is addedby use of a photonic crystal fiber.
 23. The wavelength divisionmultiplexing optical transmission method according to claim 16, wherein,in the step of causing the optical transmitter to add the predeterminedpositive dispersion amount to the WDM signal, the predetermined positivedispersion amount is added by use of a single-mode fiber.
 24. Thewavelength division multiplexing optical transmission method accordingto claim 16, wherein the optical fiber transmission path is a dispersionshifted fiber.
 25. A wavelength division multiplexing opticaltransmission method in a wavelength division multiplexing opticaltransmission system including an optical transmitter, an opticalreceiver and at least one relay node which is provided between theoptical transmitter and the optical receiver, the transmission methodcomprising the steps of: causing the optical transmitter to transmit, toan optical fiber transmission path, a WDM signal which is obtained bymultiplexing a plurality of optical signals on the optical fibertransmission path in terms of wavelength, the plurality of opticalsignals respectively having negative chirps, and the plurality ofoptical signals differing from one another in wavelength; causing eachof the relay nodes to compensate a chromatic dispersion, which thereceived WDM signal has suffered in the optical fiber in the immediatelypreceding transmission span, and to thus transmit the WDM signal to thetransmission path in the following transmission span; causing at leastone of the relay nodes to further add a predetermined positivedispersion amount to the WDM signal before the WDM signal is transmittedto the optical fiber in the following transmission span; and causing theoptical receiver to compensate a chromatic dispersion, which thereceived WDM signal has suffered in the optical fiber in the immediatelypreceding transmission span, and to thereby obtain the received WDMsignal, which includes the predetermined positive dispersion amount. 26.The wavelength division multiplexing optical transmission methodaccording to claim 25, wherein, in the step of causing the opticaltransmitter to transmit the WDM signal to an optical fiber transmissionpath, each optical signal is modulated by an LN optical modulator whichoperates with a negative chirp coefficient.
 27. The wavelength divisionmultiplexing optical transmission method according to claim 25, wherein,in the step of causing the relay node to add the predetermined positivedispersion amount to the WDM signal, the predetermined positivedispersion amount is not smaller than +100 ps/nm, and smaller than +500ps/nm.
 28. The wavelength division multiplexing optical transmissionmethod according to claim 25, wherein, in the step of causing the relaynode to add the predetermined positive dispersion amount to the WDMsignal, the predetermined positive dispersion amount is added by use ofa photonic crystal fiber.
 29. The wavelength division multiplexingoptical transmission method according to claim 25, wherein, in the stepof causing the relay node to add the predetermined positive dispersionamount to the WDM signal, the predetermined positive dispersion amountis added by use of a single-mode fiber.
 30. The wavelength divisionmultiplexing optical transmission method according to claim 25, wherein,the optical fiber transmission path is a dispersion shifted fiber.